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S/V Wyndwitch - Morgan 24
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I was reading BoatUS site today on preventing Electrical Shock Deaths and they alluded to the higher risk of ground faults occurring in marine ac or refrigeration setups....no explanation was given Enquiring Mind wants to Know....

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One of None
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Refridge & ac compressors are sealed (hermetic) the motor windings are subjected all manner of hazards, acids, heat & cold, hi & lo volts amps, unlike a motor on a air compressor in a shop. So, because of the sealed nature the motor must endure it's very common for transient current to trip gfi breakers as the motor windings get old.
 

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islander bahama 24
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That is something that sounds interest in to me as well I could see the motor shorting out and on ac systems at high voltages killing someone with a shock however all the refer systems I have worked on on boats are 12v and while it can hurt to get zapped its not usually enough to do any serious damage to your body. Is there a link to the article
 

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One of None
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It's more Like low currents that eat anodes NewHaul. not enough to kill but enough to cause problems or unreliable starts. If one knows how motors are made, one would also wonder how any don't short out. Copper wire with varnish on it was the standard since Edison and Tesla started building them.
 

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I can't find the article but they may be referring to issues with nuisance tripping on motor starting. These are not actual faults, but the GFCI unit is extremely sensitive and the large starting currents drawn by these motors can look like some type of ground fault to the GFCI.

Just as an aside, 12 V dc does not really present any type of significant shock hazard. Current magnitude (and current path) is important, but there must be sufficient voltage to push the current through the body. This is rarely possible below about 50 V, except in situations where the voltage is applied under the skin, such as things that hospitals stick in your body.

120 V ac can definitely kill you, but with 12 V dc it would be highly unlikely if not impossible.
 

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hmmm ok, how? I mean in a practical sense. Don't think I've ever considered it possible so maybe we can all learn what conditions would make it possible

Can a 12v car battery kill - Electronics Forums

Ohm's Law (again!) : Electrical Safety - Electronics Textbook

BODILY EFFECT DIRECT CURRENT (DC) 60 Hz AC 10 kHz AC
---------------------------------------------------------------
Slight sensation Men = 1.0 mA 0.4 mA 7 mA
felt at hand(s) Women = 0.6 mA 0.3 mA 5 mA
---------------------------------------------------------------
Threshold of Men = 5.2 mA 1.1 mA 12 mA
perception Women = 3.5 mA 0.7 mA 8 mA
---------------------------------------------------------------
Painful, but Men = 62 mA 9 mA 55 mA
voluntary muscle Women = 41 mA 6 mA 37 mA
control maintained
---------------------------------------------------------------
Painful, unable Men = 76 mA 16 mA 75 mA
to let go of wires Women = 51 mA 10.5 mA 50 mA
---------------------------------------------------------------
Severe pain, Men = 90 mA 23 mA 94 mA
difficulty Women = 60 mA 15 mA 63 mA
breathing
---------------------------------------------------------------
Possible heart Men = 500 mA 100 mA
fibrillation Women = 500 mA 100 mA
after 3 seconds
---------------------------------------------------------------
 

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Dirt Free
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I can't find the article but they may be referring to issues with nuisance tripping on motor starting. These are not actual faults, but the GFCI unit is extremely sensitive and the large starting currents drawn by these motors can look like some type of ground fault to the GFCI.

Just as an aside, 12 V dc does not really present any type of significant shock hazard. Current magnitude (and current path) is important, but there must be sufficient voltage to push the current through the body. This is rarely possible below about 50 V, except in situations where the voltage is applied under the skin, such as things that hospitals stick in your body.

120 V ac can definitely kill you, but with 12 V dc it would be highly unlikely if not impossible.
The factor deciding the effects of the AC and DC current is the path the current takes through the body. If it is from the hand to the foot, it does not pass through the heart, and then the effects are not so lethal.

However DC current will make a single continuous contraction of the muscles compared to AC current, which will make a series of contractions depending on the frequency it is supplied at. In terms of fatalities, both kill but more milliamps are required of DC current than AC current at the same voltage.

If the current takes the path from hand to hand thus passing through the heart it can result in fibrillation of the heart. Fibrillation is a condition when all the heart muscles start moving independently in a disorganized manner rather than in a state of coordination. It affects the ability of the heart to pump blood, resulting in brain damage and eventual cardiac arrest.

Either AC or DC currents can cause fibrillation of the heart at high enough levels. This typically takes place at 30 mA of AC (rms, 60 Hz) or 300 - 500 mA of DC.

This quote From Bright Hub Engineering but there are numerous other sources ...

Effects of an AC or DC Currents on the Human Body
The three basic factors that determine what kind of shock you experience are the amplitude of the current, the duration of the current passing through the body, and the frequency.

Direct Currents actually have zero frequency, as the current is constant. However, there are physiological effects during electrocution no matter what type of current.Though both AC and DC currents and shock are lethal, more DC current is required to have the same effect as AC current. For example, if you are being electrocuted or shocked 0.5 to 1.5 milliamps of AC 60 Hz current is required and up to 4 mA of DC current is required. For the let-go threshold in AC a current of 3 to 22 mA is required against 15 to 88 of DC current.
 

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One of None
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AC and DC Electric Shock Effects Compared
Either AC or DC currents can cause fibrillation of the heart at high enough levels. This typically takes place at 30 mA of AC (rms, 60 Hz) or 300 - 500 mA of DC.

Though both AC and DC currents and shock are lethal, more DC current is required to have the same effect as AC current. For example, if you are being electrocuted or shocked 0.5 to 1.5 milliamps of AC 60 Hz current is required and up to 4 mA of DC current is required. For the let-go threshold in AC a current of 3 to 22 mA is required against 15 to 88 of DC current.


Now on a 24 volt boat.....But who has actually been killed by boat or auto 12 volt DC? I you were killed by it please speak up! :D:eek:
 

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S/V Wyndwitch - Morgan 24
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Discussion Starter · #10 ·
Thanks for your responses. The boats us article related to ac and fresh water environments . I was very surprised the low mili amperage which can be fatal. 1/3 of what it takes to power a 60 watt bulb. It also pointed out many salt water creeks are actually brackish OE fresh near their heads...I can think of all sorts of locations here where swimming should be avoided and of course after heavy rains the effect could be wide spread.

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hmmm ok, how?
Not sure what your question is, but I think we're basically in violent agreement. In the US, the basic electrical safety standard (NFPA 70E) cuts off at 50 V. Below that, shocks may be felt, but are not life-threatening. So I almost said it was impossible to have a shock hazard at 12 V. But the Internet being the Internet, if I said "impossible", someone would have an anecdotal story about Uncle Ned who committed suicide using a 12 V lantern battery. But also these voltage limits assume contact with skin. Once an electrode is in the blood stream, the safe voltage level goes down drastically. Skin resistance might be 100 kilo-ohms, but internal body resistance is only about 500 ohms. If you have a cardiac catheter, it does not take much voltage difference to cause a major problem hence all of the special electrical requirements in hospitals, especially ICU beds and surgeries.

Anyway, bottom line, I don't worry about shock hazards with 12 V dc.

Happy New Year,

Dave
 

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One of None
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Dave, I said, if you take the context of it. "hmmm ok, how? I mean in a practical sense. Don't think I've ever considered it possible so maybe we can all learn what conditions would make it possible"

We rewired my battery banks, and more this summer and it never crossed my mind that 12 volts could kill. Now we are hearing (reading) otherwise.

Happy new year!
 
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